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Continental Slope Evolution in Tertiary: Northwest Australia

Abstract

Slope-channels provide an excellent proxy for sediment generation and movement in the inboard shelf, as well as an excellent proxy for sediment transport rates from shelf to deep basin repositories. However, unless slope structure intervenes, few plays develop in the slope. Thus, the slope depositional system is generally seen as a bypass region and little attention is paid to this system. However, the nature of the geomorphology of the slope can often provide clues to the nature of deepwater deposits and the shear volumes of sediment that are moving basinward. Recent discoveries in the Browse basin of northwest offshore Australia provide impetus for data abundance and interest in evaluating the temporal changes in sediment movements from shelf to slope. Seismic geomorphologic interpretation of a large (10,000 sq. km2) 3D seismic data volume was integrated with available wells and Arc-GIS database of mined publications to assess temporal changes in the manner in which sediment transfer happens from shelf to basin, as well as changes in sediment character and volumes. Results of observations show that the older Tertiary strata (Paleocene, Oligocene, Eocene) have very well-defined, moderate sinuosity deepwater channel levee systems feeding significant fans in the western portions of the study area. Slope systems are point sourced with large (kilometer-scale) feeders headwardly developed into the shelf-to-slope break. These feeders have well developed tributary canyons. Locations of these canyons and channel-levee systems are influenced by accommodation sinks located over deeper structuring of Mesozoic strata. Transition to a mixed clastic-carbonate system is reflected by massive progradation of deltaic systems that become dominant in the basin. By this time, submarine topography has been healed. Slope systems are line-oriented slope-gullied systems with little accumulation in the gullies, but rather hundreds of small bypassing incisions moving sediment from prograding topsets to foresets to toesets and into base of slope locations. The result is a base of slope apron rather than the previously seen large leveed channel fed fans. Transition to carbonate dominated margin occurred in the later Neogene